OPTICAL GUIDE AND MEASUREMENT APPLIANCE

Abstract: This invention is a method and apparatus for visualization of a sagittal section through a transparent optical medium proximate the inferior surface (6) of a lens/mirror assembly (2). Lens compensation means and/or mirror compensation means provide astigmatic correction for images arising along a portion of the sagittal section. For radial keratotomy, illumination of the incision by scleral scatter, anterior chamber retro-illumination, and/or self illumination of a corneal scalpel blade (102) aid in visualization of the scalpel blade within the corneal stroma. Direct visual estimates may be made of blade penetration depth into the stroma, as well as the corneal thickness which is not incised. Other embodiments of the invention facilitate estimation of the depth and degree of relative opacity in optical media resulting from, for example, an incision by a directed energy beam or photo-opacification of tissue.

Description: Optical Guide and Measurement Appliance This application is a continuation-in-part of Application Serial No. 08/151,996, filed November 15, 1993. BACKGROUND Field of the Invention The present invention relates to methods and apparatus for guiding incisions and measuring relative opacities within substantially transparent optical media, including human and animal tissues. Apparatus and Methods for Corneal Incisions The cornea is a substantially transparent, relatively tough five-layer membrane, the thickness of which varies about a nominal value of about 0.5mm. Beginning with the external corneal surface, the cornea comprises the anterior corneal epithelium (continuous with that of the conjunctiva), the anterior limiting layer (Bowman's membrane), the substantia propria or corneal stroma, the posterior limiting layer (Descemet's membrane), and the endothelium of the anterior chamber (keratoderma). Attempts to incise some or all of these layers frequently result in wrinkling, dimpling or other distortions of the corneal surface which make the incision depth, direction and extent hard to control precisely. Among the most difficult corneal incisions to perform properly are those for lamellar keratoplasty (LK) and those for refractive correction by radial keratotomy (RK). Such incisions are necessarily partial- thickness, meaning the incising blade is within the corneal tissue to a depth less than the corneal thickness at all points of the incision. Such incisions are preferably made under precise control with an extremely sharp scalpel blade of, e.g., diamond or surgical stainless steel. To control and limit the incision depth in the corneal tissue, a trephine or analogous instrument may be used to hold and/or guide the blade. A variety of trephines have been developed to facilitate partial-thickness corneal incisions, some of which provide means to compensate for variations in corneal thickness. Such compensation is especially desirable during RK because the preferred corneal incision depth may exceed 95% of the corneal thickness at every point of the incision. Even when using a trephine, however, extreme care must be taken to avoid corneal perforation and the damage to the underlying iris which often follows perforation. Using conventional pachymetry techniques to achieve relatively deep corneal incisions while avoiding perforation requires exceptionally accurate and closely- spaced measurements of corneal thickness, as well as a reliable indicator of blade cutting depth. The latter problem is particularly acute but is usually addressed through some indirect measurement. One approach is analogous to that used during goniotomy, wherein the surgeon strives to keep at least the back four fifths of the scalpel blade visible (e.g., through a mirrored goniolens or gonioprism) to avoid accidental goniopuncture. While this is a conservative approach to goniotomy, however, a similar degree of precision in RK would predictably lead to substantially unsatisfactory refractive correction in an excessive number of patients. In fact, for reliably acceptable results in RK, the accuracy in estimating both blade cutting depth and corneal thickness would have to be within plus or minus about 5 microns. Unfortunately, such accuracy has not been consistently demonstrated in practice with commercially available instruments. Thus, the probability of achieving ideal refractive correction following RK is unavoidably lowered, and the probability of corneal perforation with its attendant morbidity remains relatively high. An additional factor complicating RK surgery is that because of its toughness, the cornea is generally slightly displaced away from the blade during the incision. In fact, the blade depth is commonly set from about 20 to 35 microns deeper than the corneal thickness (as estimated by pachymetry) in an effort to produce partial thickness corneal incisions of the desired depth. Several U.S. Patents describe apparatus to be used in this procedure, but the method relies excessively on the experience and judgment of the surgeon and the accuracy of pachymetry for optimal blade depth adjustment. Even laser interferometry, though potentially more accurate than ultrasonic pachymetry, is not completely satisfactory because the corneal thickness is estimated at points other than those directly under the scalpel blade tip. Additionally, incision depth estimates using conventional pachymetry techniques lack compensation for the real-time variability of local corneal distortion caused by depression of the corneal surface under a blade tip. For example, U.S. Patents No. 4,815,463 to Hanna and No. 4,724,837 to Gannon, both incorporated herein by reference, describe estimation of the appropriate incision depth through use of templates. The device of the '463 patent uses a rigid template lens that fits against the surface of the cornea, molding it to the shape of the lens. The template lens, having a plurality of slots for guiding the blades, fits within a conical base member which is placed over the patient's eye. The apparatus uses a vacuum to secure the cornea to the template lens, and the blades are integrated directly into the device, being adjusted to provide an incision of predetermined depth as estimated by pachymetry or other means. Analogously, the '837 patent discloses an apparatus consisting of a cylindrical tubular housing, the lower end of which has a rigid multi-slotted incision guide template that fits against the cornea so the cornea (temporarily) conforms to the shape of the template. Scalpel blades are integrated into the device, a spring-loaded plunger bringing the blades into contact with the cornea so that the initial incision depth is set by adjusting the plunger stroke length. A second multi-slotted template is aligned with the corneal template to vary incision depth along the length of the cut. By changing the thickness of the second template, the blade depth can be varied along the length of the incision, but only in a predetermined manner. Other apparatus for prospectively guiding and/or controlling blade position and depth during corneal incisions is described in U.S. Patents No. 4,688,570 to Kramer et al, No. 4,619,259 to Graybill et al, No. 4,526,171 to Schachar, and No. 4,406,285 to Villasenor et al, all incorporated herein by reference. None of the devices described in the above U.S. Patents, however, permit accurate real-time monitoring of both RK incision depth and the corneal remainder i.e., the thickness of uncut cornea directly under the incising blade tip). Were accurate real-time estimation of both the incision depth and the corneal remainder thickness technically feasible, a surgeon could provide additional blade control which would increase the efficacy and safety of RK and keratoplastic surgery, while multaneously reducing the risk of complications due to corneal perforation. Direct Observation of Corneal Incisions Goniolenses, gonioprisms and certain other diagnostic and treatment lenses allow a surgeon to visualize structures within the eye, including portions of the anterior chamber near the angle (gonio- is a combining form denoting relationship to an angle). Structures observed using goniolenses or gonioprisms, however, commonly lie between the ciliary body and the cornea (e.g., the trabecular meshwork) but not within the cornea. Further, optical compensation elements added to a goniolens or gonioprism to reduce aberrations in images of the trabecular meshwork do not facilitate imaging within the corneal stroma. See, for example, U.S. Patents No. 4,506,962 to Roussel, No. 4,598,984 to Rol, No. 4,664,490 to Rol, and No. 5,252,998 to Reis et al, all incorporated herein by reference. Notwithstanding the optical characteristics (with or without compensation elements) of any of the above devices, they are not clinically useful for guiding RK incisions in a meridional plane in the corneal stroma. Visualization of the course of such an incision, if achievable at all with a goniolens, gonioprism, or analogous diagnostic or treatment lens, would be generally yield low-intensity images with substantial astigmatic aberration. Clinically useful control of RK and LK incisions, of course, requires bright images without substantial aberration to facilitate the precise measurements on which their efficacy depends. Thus, if attempts were made to use existing optically compensated diagnostic or treatment lenses for viewing the corneal stroma, they would tend to produce a substantially dark field because light directed through such lenses to illuminate eye structures falls almost exclusively posterior to the cornea. Even though the lenses are designed to admit light into the eye (as from a Nd-YAG or argon laser), the light thus admitted is intended to strike more posterior structures, not the cornea. Attempts to correct this light path by tilting the internal mirror would generally not have the desired illuminating effect because the redirected light would then tend to be totally internally reflected within the lens and thus not reach the cornea. Further, even if sufficient scattered light were present to view intracorneal structures, the images would be subject to aberrations so severe as to substantially preclude their clinical use. Thus, the optical compensation which makes a goniolens, gonioprism or other conventional diagnostic or treatment lens effective in portions of the eye posterior to the cornea differs in both character and amount from that which would allow real¬ time incision depth estimates by direct observation along the length of a meridional RK incision. Further, no currently available corneal lens provides a means for incising corneal tissue proximate the inferior surface of the lens while the incision and adjacent corneal tissues (including anterior and posterior corneal surface edges) are directly observed through the lens. SUMMARY OF THE INVENTION The present invention, in contrast, comprises new and improved methods and apparatus for guiding and controlling partial-thickness corneal incisions (as for RK and LK) through simultaneous real-time visualization or analogous optical monitoring of illuminated intracorneal structures indicative of both incision depth and corneal remainder thickness. In addition to facilitating RK and LK, analogous methods and apparatus of the invention may be applied to precisely incise by blade or scanned energy beam any substantially transparent tissue or other optical medium to which the apparatus may be physically applied. Indeed, when any substantially transparent optical medium (i.e., a medium capable of transmitting at least a portion of incident electromagnetic radiation) is incised, the result is increased attenuation and/or altered refraction of light in the medium (i.e., creating a relative opacity in the medium). The opacity, in turn, may be observed and/or measured in real time or at a later time using one or more embodiments of the invention. The invention comprises an illuminating lens-mirror assembly for visualizing images (substantially in cross-section) within optical transmission medium proximate the lens-mirror assembly. The lens-mirror assembly is termed a keratoscopic lens when the inferior lens surface is substantially concave so that one may visualize images of intracorneal structures in a cornea proximate the keratoscopic lens inferior surface. The lens-mirror assembly itself comprises a lens comprising an optical axis, an inferior surface, a superior surface, at least one internal mirror, and optical compensation means to substantially correct aberrations in the images of intracorneal structures. Optical compensation means, in turn, may comprise mirror compensation means and/or lens compensation means. A scalpel slot may extend through the lens from the inferior surface to the superior surface, and/or illuminating means may be coupled to the lens for illumination of optical transmission medium (e.g., a cornea) which may be proximate the lens inferior surface. The illuminating means, when present, preferably comprises at least one substantially monochromatic light source and may provide substantially retroillumination or substantially scleral scatter illumination for a cornea to which the inferior lens surface is proximate. Each internal mirror may comprise mirror compensation means to increase resolution in images formed substantially of light rays oblique with respect to the lens optical axis, the mirror compensation means comprising a substantially spherical, substantially cylindrical, or substantially toroidal reflecting surface. Analogously, the superior lens surface may comprise lens compensation means to increase resolution in images formed substantially of light rays oblique with respect to the lens optical axis, the lens compensation means comprising a substantially spherical, substantially cylindrical, or substantially toroidal refracting portion of the superior lens surface. The invention also comprises a scalpel which itself comprises an elongated hollow scalpel shaft having a longitudinal axis, a first end, and a second end, and blade adjustment means within and adjustably coupled to the scalpel shaft for extending and retracting a blade with respect to the shaft second end substantially parallel to the shaft longitudinal axis. A blade is coupled to the blade adjustment means proximate the scalpel shaft second end, and displacement means are coupled to the blade adjustment means and the scalpel shaft to impart substantially longitudinal movement to the blade adjustment means with respect to the scalpel shaft for extending and retracting the blade with respect to the shaft second end substantially parallel to the shaft longitudinal axis. Local fiber optic means couple electromagnetic radiation to the blade, and input fiber optic means (spaced apart from the local fiber optic means by a fiber optic gap between the two within the scalpel shaft) deliver electromagnetic radiation to the local fiber optic means. The displacement means may comprise a pneumatic motor, a pico motor, a brushless direct-current motor, or a stepper motor, and the input fiber optic means may comprise at least one laser diode and/or at least one remote fiber optic extending from the local fiber optic means to an electromagnetic radiation source outside of the scalpel shaft. The blade may comprise steel, diamond, and/or fluorescent material and the electromagnetic radiation coupled to the blade by the local fiber optic means may travel substantially within the blade (when the blade is substantially transparent, e.g., diamond) and or may travel outside the blade but proximate to and substantially parallel to it (whether the blade is substantially transparent or substantially opaque). The scalpel may also comprise feedback position control means, wherein the blade adjustment means is coupled through the feedback position control means to the power transmission means for feedback control of the blade adjustment means. Note that the words "light" and "optical" herein pertain to electromagnetic radiation of any wavelength, not just visible wavelengths, and optical transmission media as used herein refers to materials through which electromagnetic radiation (which may include visible light) can pass. Such passage of radiation denotes transparency of the medium, but the amount of radiation passing is typically measured relative to that introduced so that absolute transparency (i.e., 100% transmission) is not required. A relative opacity in the medium attenuates or otherwise alters transmission of radiation compared to that transmitted along a similar path in the medium without the opacity. Note also that one or more wavelengths and/or intensities of electromagnetic radiation may be chosen to illuminate the optical medium to enhance, for example, medium transparency and/or the visibility of a scalpel blade or an area of relative opacity. Thus, one or more substantially monochromatic sources of illumination (e.g. , lasers or laser diodes) may be used to increase the efficacy of real-time visualization or analogous optical monitoring. Superior visualization and/or monitoring may in turn be employed to improve control of tissue photoalteration (i.e., photocoagulation, photoablation and/or photodisruption, as by a directed energy beam), or to guide the path of an incision. Where direct visualization of an object or structure is desired (e.g. , a knife blade in an incision), it may be accomplished with the aid of image display or enhancement systems responsive to electromagnetic radiation within or outside of the visible spectrum. Such radiation may be used, for example, to observe the course of a scanned energy beam or other tissue-altering modality (e.g., electrocautery) in relatively thin tissue sheets. Changes in relative tissue opacity due to tissue alteration may alternatively be detected visually or by analogous optical means of the present invention with the appropriate choice of illuminating radiation frequency, illuminating source placement, and optical coupling media. The physical extent and quantitative optical effects of such changes in opacity may also be accurately estimated by using the measurement appliance of the present invention. Where opacity is directly related to the degree of tissue photoalteration, the adequacy of such photoalteration to accomplish some therapeutic goal (e.g., treatment of endometriosis) may then be assessed in a quantitative manner both as to the depth and degree of the photoalteration. The measurement appliance of the present invention uses electromagnetic radiation introduced into one side (the first side) of sheet or bulk optical transmission medium through an electromagnetic radiation port and appropriate optical coupling medium. At least a portion of the radiation is directed at the region of the optical transmission medium which contains (or will contain) an incision and/or other relative tissue opacity. Radiation transmitted through the opacity or through the optical transmission medium lying between the opacity and the second side of the transmission medium passes to an electromagnetic radiation receiver which is optically coupled (through an electromagnetic radiation coupler which may comprise a lens- mirror assembly, as above) to the transmission medium and spaced apart from the radiation port. In certain preferred embodiments for use with sheet optical transmission media, the radiation may experience total internal reflection at the second side of the sheet. Regardless of its path through the optical medium, the intensity and/or direction of received electromagnetic radiation are detected, measured and scaled to obtain, for example, estimates of the depth, length and/or degree of the relative tissue opacity which extends into the sheet transmission medium from the first side. A depth estimate may apply, for example, to the depth of a scalpel blade in tissue (the optical transmission medium), while depth, length and degree estimates may apply, for example, to photoalteration induced in abdominal tissue surfaces for treatment of endometriosis. To further improve visibility of a scalpel blade in an incision, contrast agents and optical coupling agents may be specially selected for certain embodiments of the invention. Dyes and/or other light-responsive substances (e.g., fluorescent compounds) may be applied to or otherwise incorporated within scalpel knives during manufacture or they may be applied locally to knives or optical media to increase contrast. An optical coupling fluid may be chosen so that its index of refraction either facilitates or prevents total internal reflection at an optical interface, thus, for example, allowing direct visualization of a knife blade in corneal tissue within a predetermined range of distances from a lens inferior surface and from the optic zone of the cornea. Additional visual enhancement may be obtained in certain embodiments (e.g., for corneal incisions) by injecting a fluid (e.g., a gas such as air or nitrous oxide, or a liquid such as glycerin, chondroitin sulfate or methyl cellulose) in a corneal incision to effectively delaminate the corneal tissue along a uniform plane and at an appropriate depth within the cornea. The composition and osmolarity of delaminating fluid may be chosen to facilitate fluid removal and/or absorption at the end of the surgery. Note also that a lens-mirror assembly of the present invention can facilitate focussing a directed energy beam on a tissue layer within the cornea for tissue ablation in pursuit of refractiv -.orrection of the cornea. Through appropriate ce of dyes, coupling fluids, contrast agents and/or delaminating fluids, together with one or more illuminating radiation frequencies and source placements, the present invention allows real-time monitoring of incisions and relative opacities in optical media, and of corneal or other tissue layer separation. Note that such monitoring may include human or machine vision or both, and may be used to guide the hand of a surgeon and/or to control the incising blade depth or scanned energy beam power level and/or delaminating fluid flow in a substantially automatic feedback control system. By optimal choice of indices of refraction of lens elements and coupling fluids in the present invention, reflections at tissue interfaces may be controlled to produce intraocular illumination and image stability not currently available with other contact lens systems (e.g., goniolenses or gonioprisms). Preferred embodiments of the apparatus of the vention as it relates to RK comprise a novel incision guide (itself comprising a lens-mirror assembly) and corneal illumination means which may or may not be coupled to the lens. A scalpel (itself comprising an internally and/or externally illuminated blade) or a directed energy beam source may be used with the illuminating lens-mirror assembly to reveal the effective blade or incision (or relative opacity) depth in the tissue through measurement of received electromagnetic radiation and/or simultaneous visualization of both the corneal incision and the incising portion of the scalpel blade during RK. In embodiments comprising an illuminated scalpel blade of substantially transparent material (e.g., diamond), an additional reference for measuring corneal remainder thickness may comprise the spot of light transmitted from the blade tip and reflected at the posterior surface of the cornea. Notwithstanding the choice of various blade- position references in certain embodiments of the invention, accurate real-time estimates of the effective incision depth are readily obtained and allow a surgeon to make substantially continuous depth adjustments to maintain a desired corneal remainder thickness. Note that the substantially cross-sectional view of a corneal incision and adjacent tissue provided by a lens-mirror assembly of the present invention differs from that provided by a goniolens or gonioprism in that the latter devices are generally designed to provide only a surface view of structures spaced apart from the corneal surface (e.g. , the trabecular meshwork or anterior chamber angle). Any cross- sectional view of the corneal stroma, especially the portion outside of the optic zone, through a conventional goniolens or gonioprism would tend to be relatively poorly illuminated and relatively narrow in scope. The image may also be relatively unstable in certain lenses because of the substantial decentering required with respect to the cornea. Additionally, lack of scleral scatter or retroillumination, and the absence of correction for a series of images obtained along a corneal meridian make images obtained through a goniolens or gonioprism unsuitable for the precise incision depth estimates needed for successful RK. Inclusion within the incision guide of precise positioning means (i.e., a lens- mirror guide assembly) for locating the lens-mirror assembly on the corneal surface further distinguishes certain embodiments of the present invention. The lens-mirror guide assembly allows rotation of the lens-mirror assembly within the guide assembly while facilitating vacuum fixation of the guide assembly to the eye, thus providing for controlled rotation of the lens-mirror assembly with respect to the corneal surface. Mechanical rotational stops, spacer blocks, guide pins, locks and/or detent position adjustments for altering the rotational position of the lens-mirror assembly within the guide assembly further enhance the surgeon's control in determining the position, depth, direction and extent of each corneal incision. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A schematically illustrates a plan view of a lens-mirror guide assembly with a lens-mirror assembly (i.e., a keratoscopic lens) in place. Figure IB schematically illustrates the frontal cross-sectional view indicated on Figure 1A. Figure 1C schematically illustrates the side cross-sectional view indicated on Figure 1A. Figure ID schematically illustrates a side elevation of the lens-mirror assembly portion of the lens-mirror guide assembly of Figure 1A. Figure IE schematically illustrates a plan view of an arcuate guide piece intended to fit within the scalpel slot of the lens-mirror assembly of Figure ID. Figure IF schematically illustrates an end elevation of an arcuate guide piece intended to fit within the scalpel slot of the lens-mirror assembly of Figure ID. Figure 1G schematically illustrates the indicated side cross -sectional view of the arcuate guide piece of Figure ID. Figure 2A schematically illustrates a plan view of the arcuate slot ring of the lens- mirror guide assembly of Figure 1A. Figure 2B schematically illustrates the side cross-sectional view of the arcuate slot ring indicated on Figure 2A. Figure 3A schematically illustrates a plan view of the keratoscopic lens holder of the lens-mirror guide assembly of Figure 1A. Figure 3B schematically illustrates the side cross-sectional view of the keratoscopic lens holder indicated in Figure 3A. Figure 4A schematically illustrates a plan view of the upper guide assembly housing of the lens-mirror guide assembly of Figure 1A. Figure 4B schematically illustrates the side cross-sectional view of the upper guide assembly housing indicated in Figure 4A. Figure 5A schematically illustrates a plan view of the lower guide assembly housing of the lens-mirror guide assembly of Figure 1A. Figure 5B schematically illustrates the side cross-sectional view of the lower guide assembly housing indicated in Figure 5A. Figure 6A schematically illustrates a cross-sectional view of ray tracings representing use of the keratoscopic lens to image a knife point located at the edge of a 3.0 mm optic zone and 600 μ (microns) deep in a cornea which is proximate the inferior surface of the keratoscopic lens. Figure 6B schematically illustrates a cross-sectional view of ray tracings representing use of the keratoscopic lens to image a knife point located at the edge of a 5.0 mm optic zone and 600 μ deep in a cornea which is proximate the inferior surface of the keratoscopic lens. Figure 6C schematically illustrates a cross-sectional view of ray tracings representing use of the keratoscopic lens to image a knife point located at the edge of a 7.0 mm optic zone and 600 μ deep in a cornea which is proximate the inferior surface of the keratoscopic lens. Figure 7A schematically illustrates an isometric view of intensity values of light rays forming the knife point image of Figure 6A. Figure 7B schematically illustrates an isometric view of intensity values of light rays forming the knife point image of Figure 6B. Figure 7C schematically illustrates an isometric view of intensity values of light rays forming the knife point image of Figure 6C. Figure 8A schematically illustrates an isometric view of intensity values of light rays forming a knife point image analogous to that of Figure 6A except through the use of a Zeiss gonioprism instead of a keratoscopic lens. Figure 8B schematically illustrates an isometric view of intensity values of light rays forming a knife point image analogous to that of Figure 6B except through the use of a Zeiss gonioprism instead of a keratoscopic lens. Figure 8C schematically illustrates an isometric view of intensity values of light rays forming a knife point image analogous to that of Figure 6C except through the use of a Zeiss gonioprism instead of a keratoscopic lens. Figure 9 schematically illustrates an isometric view of ray tracings representing use of a Roussel lens to attempt to image a knife point located at the edge of a 7.0 mm optic zone and 600 μ deep in a cornea which is proximate the inferior surface of the Roussel lens. Figure 10 schematically illustrates a partial cross-sectional view of a scalpel comprising an illuminated diamond knife blade. DETAILED DESCRIPTION Correction of Corneal Curvature Radial keratotomy (RK) is a surgical procedure to reduce the corneal central curvature, thereby reducing myopia. Embodiments of the lens-mirror assembly of the present invention having an inferior surface curvature approximating that of the cornea are designed to improve the reproducibility and accuracy of radial and astigmatic keratectomy procedures and technically simplify them. The principle effect of RK results from one or more radial incisions of varying length and of approximately 95+% corneal thickness in depth. The number of equally spaced radial incisions numbering (e.g., 4, 8 or 16), and the diameter of a clear central optic zone which is spared, depend on the sex, age, original corneal curvature and the desired myopic correction for each patient. Astigmatic keratectomy can accompany radial keratectomy or can be performed as a primary refractive procedure. Instead of radial incisions, astigmatic keratectomy reduces corneal astigmatism by placement of tangentially oriented incisions approximately 2.2 mm in length and 95+% corneal thickness in depth at various circumferentially placed optic zones. These incisions produce meridionally weighted flattening of the corneal curvature. Both radial and astigmatic keratectomy procedures typically require determination corneal thickness (and thus, of blade cutting depth) in several locations by use of an ultrasonic pachymeter. Blade depths approximately 98% of corneal thickness are usually required to produced desired corneal flattening. The tolerance for blade depth accuracy may be only 5 microns, so use of an ultrasonic pachymeter to set blade depth can result in large errors in desired incisional depth. Perforations of the cornea are common; moreover, shallow radial or transverse incisions often reduce the effectiveness of the procedures. In fact, the inaccuracy of this step in these procedures is exemplified by the fact that the blade depth is usually set at 20 to 35 microns deeper than the ultrasonically measured corneal thickness to produce the desired effect. Although radial and astigmatic keratectomy are facilitated by various radial, transverse and optic zone markers, the procedures are currently often done free handed. Errors in incision placement, incision length, and incision direction are common, and radial incisions may be inadvertently extended into the optic zone. Moreover, transverse corneal incisions often inadvertently extend into adjacent radial incisions, causing irregular astigmatism and wound gape. Improved accuracy of blade depth estimates and mechanical control of incision placement could increase the overall predictability and effectiveness of corneal correction procedures and possibly reduce overall morbidity. The present invention can improve the accuracy of blade depth setting and includes means to mechanically control the placement, length and direction of radial and transverse corneal incisions. These improvements are achieved through visual or analogous optical observation of the tip of the cutting blade and the proximate corneal tissue within a corneal cross- section to estimate corneal incision depth in vivo. Analogously, the invention can also provide direct visual or other optical evidence of the depth of any relative optical opacity in an optical transmission medium. As schematically illustrated in Figures 1 A-5B, the present invention facilitates direct observations of cross-sections in optical transmission media through a combination of methods and apparatus unique to such observations. Included in the methods are those related to transmitting and magnifying images originating from a meridional, cross-sectional plane within a cornea and reducing aberrations in the magnified images to clinically acceptable levels. Transmission of the images into and through the lens-mirror assembly (which substantially corrects most of the aberrations) requires an empirically-determined choice of appropriate optical coupling medium between the cornea and the inferior surface of the lens-mirror assembly. In a preferred embodiment of the invention, a lens-mirror assembly with a radius of curvature of about 7.4 to 10.0 mm (preferably about 9 mm) on its inferior surface is placed on the corneal surface, thus forcing the cornea to conform to the lens. The lens-mirror assembly, in turn, contains an internal mirror, angled at about 45 to 75 degrees (preferably about 60 degrees) to the tangent of the central cornea, to permit observation of the incision through the (compensating) superior lens surface. This provides a view of a cross-section of the cornea which includes the cutting blade as it projects beneath the lens inferior surface through a scalpel slot which extends between the superior and inferior lens surfaces. The blade holder (e.g., a scalpel) travels substantially radially along a groove in the scalpel slot which follows the curvature of the (conforming) cornea. Thus, radial corneal incisions of consistent depth are produced. Astigmatic incisions are performed by orienting the cutting blade transversely in the radial groove at the proper distance from the optical axis of the cornea. Visualization of the blade depth is again via the internal mirror and the optically compensating superior lens surface. The incision is produced by rotation of the lens and blade (within the lens-mirror assembly holder) around a central axis which is preferably aligned with the corneal visual axis. This rotation is limited by mechanical stops to produce defined length arcuate corneal incisions. The lens-mirror assembly is preferably constructed of tempered glass or injection molded plastic, preferably in two pieces which intersect at a (preferably) 60 degree angle parallel to the radial groove and slot. Alternatively, a one-piece lens- mirror assembly can be produced if the portion of the lens-mirror assembly behind the desired mirror surface (as viewed from the slot) can be removed, leaving a polished glass-air or plastic-air interface in the shape of the desired mirror surface. The glass-air or plastic-air interface then can act as a mirror by totally internally reflecting (TIR) light from the incision, the reflected light then passing superiorly within the lens-mirror assembly and out its (corrected) superior surface. If a metal surface mirror is preferred to a TIR surface, aluminization of the intersection of a two-piece assembly creates an internal mirror which can function substantially as the above TIR surface. Because the lens material (either glass or plastic) has a higher index of refraction than the tear film-cornea, illumination of the cornea from a light source from above the lens would produce total internal reflection as viewed through the internal mirror for visualization of objects within the corneal stroma. Since internal reflection occurs only on passage of light from a high index of refraction media into a low index of refraction media, the source of illumination needs to come from beneath the lens-mirror assembly. To accomplish this retroillumination, a fiber optic cable may be placed within the lens-mirror assembly and shielded from the radial groove by the mirrored interface. The output end of the fiber optic cable preferably is positioned near the limbus to produce sclerotic scatter type internal corneal illumination. In embodiments where the diamond knife blade making the incision is internally illuminated, the above retroillumination becomes unnecessary because of the light which is emitted from the blade. The lamp source for either type of illumination can be a halogen light box. To assist orientation, stabilization and concentric movement around the desired corneal axis, the lens-mirror assembly is incorporated into a holder and guide device. The guide device can be a metallic or plastic unit constructed of, for example, surgical grade stainless steel, titanium, acrylic, polyvinylchloride, Teflon or similar material, and preferably comprises three concentric cone structures, each angled at 60 degrees to the iris plane in the eye. The innermost cone supports the lens-mirror assembly, and, by way of detent ball and spring components or a spring clip, it can rotate the lens about a central axis and click into position in up to sixteen equally spaced radial meridians around the defined central axis. The second cone may be coupled to a disc-like housing by way of a threaded joint, or the lens may be allowed to move passively superiorly and inferiorly. The threaded joint, if used, adjusts the vertical height of the lens and helps improve cornea-to-lens contact during incision with the knife blade. The outside cone is preferably sealingly coupled to the middle cone structure to form a vacuum chamber when the guide is placed into contract with the conjunctiva. Vacuum through a side port in the outside cone structure is provided by a syringe or vacuum pump at pressure of about 15 to about 200 mmHg (preferably about 100 mmHg). The central lens holder (inner cone structure) is suspended within the outer housing and outer cone structures by a flanged lower bearing surface directly supported by the disc shaped outer housing. This lens holder has 16 equally spaced recesses which allow the lens slot to be positioned in 16 radial positions over the cornea. The lens-mirror assembly fits from above into the taper of the lens holder with a vertically oriented bearing surface to allow superior and inferior lens displacement which is confined superiorly by a flange on the lens holder and confined inferiorly by the taper of the lens holder. The lens-mirror assembly is stabilized in the holder by a plurality of small pin-like projections confined within a groove in the wall of the lens holder assembly. Additionally, the lens-mirror assembly includes a radial projection from the periphery of the scalpel slot which on assembly lies within and closely fits a corresponding keyway in the lens holder. The interface of the lens- mirror assembly projection within the lens holder keyway ensures that the scalpel slot in the lens-mirror assembly can e properly oriented and stabilized in position with respect to the lens holder. To facilitate tangential astigmatic cuts the circular rotational movement of the groove and slot of the lens-mirror assembly must be confined to opposite arcs of limited, defined meridians. Moreover, it is necessary to avoid tangential arcuate cuts from intersecting adjacent radial cuts. To define the limited movement circularly, slots are placed on the upper surface of the guide housing so as to subtend a slightly smaller angle with the base being the lens-mirror assembly center, than the detent index position. A similarly oriented slot is positioned about 180 degrees away, on the other side of the guide housing. To limit the movement of the lens holder assembly between the two adjacent detent recesses, an extension to the outer vertical surface of the astigmatic guide assembly is added, which inserts into and is stopped by the slots. To effect an arcuate tangential cut in a desired meridian, the lens housing is rotated into a position in which the center of the desired arcuate slot straddles the desired meridian. The arcuate guide assembly is inserted into the knife slot and the detents for indexing are released. The knife blade is oriented transversely within the knife groove with the arcuate guide at the proper distance from the lens center. Rotation of the lens holder between the adjacent stops of the arcuate slots produces the desired arcuate cut without intersecting possible adjacent radial incisions. The knife depth can again be accurately determined by visualization of the tip within the tissue by use of the mirrored lens. To facilitate visualization of the knife blade tip within the corneal tissue for both radial and arcuate corneal incisions, a unique knife design is employed in certain embodiments. This knife comprises a diamond blade optically coupled to a shaft that incorporates a fiber optic which in turn is optically coupled to a light source. Light emitted from the blade will illuminate both the blade and the proximate corneal tissue, enhancing visualization. Functional Requirements of the Lens-Mirror Assembly The uniqueness of the lens-mirror assembly of the present invention in comparison to diagnostic and surgical gonioscopy and fundoscopic lens systems relate to its intended function. The function of the keratoscopic lens includes visualization of an object in a cross-sectional plane within the corneal stroma. Both gonioscopy and fundoscopic lens systems allow visualization of structures through meridional ray bundles traversing the cornea, but these systems cannot be used clinically to visualize an object in a cross-sectional plane within the cornea for several reasons. Light rays forming an image through a lens system in contact with the corneal epithelium and tear film from an object within the corneal stroma will constitute mostly skew rays with respect to the lens system. If this object to be imaged above the cornea from rays arising from multiple points along a meridian of the cornea from within the stroma, the skew rays will become more oblique as the object is moved peripherally within the stroma along this meridian. The obliqueness of the light rays produces significant astigmatic error and tangential coma not correctable by conventional gonioscopy and fundoscopic lens systems. Moreover, the meniscus lens configuration of the cornea with similar indexes of refraction in both the tear film and aqueous humor minimizes the refracting influence of the cornea. The overall cumulative refractive power exerted on light rays passing from the tear film through the cornea into the aqueous humor is approximately 5.88 diopter. When light rays exit from within the corneal stroma into the tear film, they are subjected to a relative refractive power of approximately 15 diopters. This creates a more active refractive corneal interaction in visualization of planes within the corneal stroma than in planes that require projection of rays through all surfaces of the cornea. This relative refractive activity increases obliquely induced astigmatism mentioned above. Illuminating objects and structures within the corneal stroma can also present requirements for the lens system that differ from those seen in gonioscopy and fundoscopy systems. With a lens system composed of relatively higher index of refraction material than the index of refraction of the tear film, light rays exiting the posterior lens surface into the tear film and corneal epithelium are totally internally reflected at angles that would be necessary to illuminate cross sectional planes within the corneal stroma. This means that cross sectional views are not possible when the originating illumination is transmitted through the optical system into the corneal epithelium. But this type of illumination is used in all indirect gonioscopy lens and fundoscopic lens. This is especially true in therapeutic lenses in which laser energy sources are focused through the optics of the lens system to irradiate structures within the eye. Because these lenses are not optimized to retroilluminate the cornea or create sclerotic scatter, the corneal stroma is poorly illuminated for visualization. If visualization of objects and structures in a cross sectional plane within the corneal stroma is a defined goal, the optical realities mentioned above must be considered to create a functional optical imaging system. For a knife blade moving radially along a meridian within the corneal stroma to be visualized at each point along the meridian with resolution sufficient to accurately determine its depth with respect to the endothelial or epithelial surface, it must be adequately illuminated; light rays emanating from its surface must be gathered efficiently by the optical system; and, imaging and correction of distortions, especially those resulting from the obliqueness of the light rays exiting from the knife surface across the cornea into the lens system must be accomplished. To efficiently gather skew light rays emanating from the corneal stroma from an object oriented traverse to the corneal epithelial surface, optical coupling to the corneal epithelial surface with saline or other substances approximating the. refractive index of the cornea can increase the critical angle of total internal reflection. This can allow gathering of light bundles from points along the corneal epithelial surface that lie relatively close to the object within the corneal stroma, minimizing loss through relative opacities within the corneal stroma. Once the light rays enter the coupling medium, they can enter a refractive posterior surface of a lens-mirror system, be gathered by a fiber optic bundle, or be directly reflected by one or more mirrors within the lens-mirror system up to the visualizing system. If the light rays enter a refracting surface, this surface can be spherical, aspheric (e.g. , cylindrical) or toric in nature. If the posterior surface of the lens-mirror system is spherical, astigmatism resulting from the obliqueness of the entering skew rays must be corrected subsequent to this refracting surface either by a plurality of obliquely-positioned mirrors, by a single spherical, cylindrical, or toric mirror surface, or by a corrective lens on a subsequent surface or on the exiting surface of the lens-mirror system. Obliquely placed mirror surfaces within the lens-mirror system can offset the obliqueness of skew rays forming certain views within the corneal stroma. If the posterior surface of the lens-mirror system is toric or aspheric, they can be optimized to reduce tangentially or obliquely induced astigmatism. A simple reflecting system composed of a single mirror or multiple mirrors located within the lens-mirror assembly can efficiently gather light rays exiting the corneal epithelium from a transversely placed intrastromal object; however, some astigmatic error can be present in the image formed by these rays unless optical compensation means are present within the visualization system to reduce the error. Fiber optic bundles within the lens-mirror assembly at oblique angles to the corneal epithelium can gather light rays from similar light rays and propagate them to a visualization system which could be purely optical or include video capabilities. Illuminating objects within the corneal stroma for purposes of visualization can be accomplished by creating a luminous object within the corneal stroma. Fiber optic illumination of a diamond blade by transillumination of the blade itself is one such example. However, illumination by sclerotic scatter or retro-illumination of the cornea are other means to accomplish this task. Use of induced or spontaneous fluorescence is another means of creating this luminescence of the object within the corneal stroma. The obliqueness of the rays entering the optical imaging system can also create chromatic dispersion. If a polychromatic light source is used for illumination, the imaging system can be optimized to reduce this chromatic dispersion by using doublets or triplets in the light path; however, monochromatic light sources can also be used, allowing optimization for a single wavelength through the light path. Chromatic dispersion may also be reduced in the correction of tangential coma as mentioned above and by appropriate choice of the coupling medium index of refraction as well. Preferred Embodiments of the Lens-Mirror Assembly The lens-mirror assembly of the present invention, which is preferably mounted in a lens holder, facilitates the simultaneous visualization of the cornea in cross -section (i.e., a transverse meridional view) together with that part of a blade extending beyond the concave lens surface and into the adjacent corneal stroma. The assembly comprises conventional refractive materials (e.g., tempered glass or transparent plastic such as polymethylmethacrylate, polystyrene or polycarbonate) with at least one plane or curved internal reflective surface (i.e., a mirror within the lens body), and at least one refractive (superior) surface. Correction of astigmatic errors for transverse meridional views may be accomplished through mirror compensation means and/or lens compensation means incorporated in the lens-mirror assembly. A (preferably radially directed) scalpel guide slot facilitates placement of a blade through the lens-mirror assembly in certain embodiments, thus positioning the blade tip for incision and into corneal tissue or other optical transmission medium which may be proximate the inferior surface of the lens-mirror assembly. The lens holder may in certain embodiments comprise the periphery of the lens-mirror assembly itself, or a separate structure in which the lens- mirror assembly is placed to facilitate handling and/or positioning. The reflective surfaces (e.g., aluminized, silvered or total internal reflection surfaces) within the lens-mirror assembly is (are) oriented within the assembly to allow observation through the superior surface of the lens-mirror assembly of at least a portion of the scalpel blade tip which projects beyond the inferior surface into corneal tissue. The tissue, in turn, is held substantially adjacent to the lens surface by, in preferred embodiments, a partial vacuum applied to the cornea and/or surrounding eye tissue through the lens itself, the lens holder, or a structure (e.g., a vacuum cone) coupled to the lens holder. Note that in practice the cornea is generally dimpled away from the lens-mirror assembly inferior surface by the scalpel (knife) blade. In practice, the tissue being incised may be illuminated (by scleral scatter or retroillumination) with a light source coupled to the lens-mirror assembly or its holder. For example, a fiber optic (connectable to an external light source) may be disposed within the lens-mirror assembly or its holder so as to be capable of illuminating the incision. A separate fiber optic or other light source may be coupled to the scalpel blade when, as in certain embodiments, the blade is substantially transparent or translucent. One or more separate light sources which are not directly coupled to the scalpel or lens-mirror assembly may also provide supplemental or primary illumination. Blade visibility may also be improved if at least a portion of the blade appears phosphorescent and/or fluorescent, the blade being otherwise (optionally) substantially transparent, translucent, or opaque. Blade and/or incision visibility may be enhanced in certain embodiments of the invention by application of a dye (e.g. , fluorescein) to the cornea, the dye being used to highlight the epithelial surface to help determine the depth of corneal penetration by the blade. Certain portions (e.g., depth markings, edge markings) of a blade surface may be prepared (e.g., by chemical, electrical, or energy beam etching) to preferentially allow emission of otherwise totally internally reflected light from within the substantially transparent (diamond) blade which would make the blade or certain portions thereof more visible. Note that in embodiments which include automatic monitoring means for incision depth, blade depth markings as measured from the blade tip may be coded so as to be readable by a scanned beam (e.g., as a bar code or machine readable numbers). In embodiments of the lens-mirror assembly having a radially directed guide slot, the slot is preferably substantially symmetrical about a central plane of symmetry which contains the optical axis of the lens-mirror assembly and extends throughout the entire lens-mirror assembly thickness. In certain embodiments, however, the superior or viewing surface of the lens-mirror assembly (which is opposite the concave inferior surface intended for contact with the cornea) may be asymmetrical with respect to the slot, being convex on one side of the slot and planar on the other side. Actual slot dimensions are determined by the scalpel and/or blade intended to be inserted within and guided by the slot, as well as the length and orientation of incisions desired, the physical properties of the lens material, and manufacturing considerations. In each embodiment, the slot accommodates positioning and/or sliding movement of a scalpel as required to accomplish the desired incisions, but the slot is also sufficiently close fitting to act as a precise guide for the scalpel. For guide slots in which the central plane of symmetry contains a radius from the lens optical axis, the length of the radial dimension of the slot and the distances from the radially directed slot limits to the lens optical axis are determined by the desired length of radial incisions to be made with the aid of the lens and the desired distance of each incision endpoint from the lens optical axis. In preferred embodiments, radial slot limits may be modified by one or more spacer blocks which may be (temporarily) locked in a guide slot. Note that arcuate incisions can be made by locking or otherwise placing a scalpel in the guide slot with the blade oriented to cut substantially perpendicular to the plane of symmetry of the slot within the arcuate guide (Le., tangentially oriented in a radially-directed slot) and advanced to the proper depth. Subsequent rotation of the lens-mirror assembly (with its holder) about an axis substantially collinear with the lens optical axis (preferably between mechanical detents or stops) will result in an arcuate incision of the cornea, the progress of which can be monitored and controlled by visualization in real-time through the lens-mirror assembly. In a preferred embodiment of the invention (Le., a keratoscopic lens), a lens- mirror assembly having a radial slot and a radius of curvature of the inferior lens surface of approximately 9mm is intended for placement on the anterior corneal surface, to be held in place there by a partial vacuum applied to the anterior eye surface (preferably through a vacuum cone) which is part of a base (housing) coupled through a lens holder to the lens-mirror assembly. The lens-mirror assembly contains an internal plane mirror which is oriented at an angle of about 60 degrees to a plane perpendicular to the optical axis of the lens-mirror assembly (Le., the plane of the mirror forms a minimum acute angle of about 30 degrees with a line parallel to the optical axis of the lens-mirror assembly and passing through the plane of the mirror). The lens-mirror assembly allows a surgeon to view (substantially in cross- section) the progress of an incision made by a scalpel blade held by a scalpel which slides radially within the guide slot (for radial incisions), or which is locked in a fixed tangential orientation within the guide slot during rotation of the lens-mirror assembly (and its holder) substantially about its optical axis (for arcuate incisions). In use, the blade protrudes a small distance (corresponding to the incision depth) beyond the concave lens surface, and the plane mirror is oriented so that a line within the mirror plane is substantially parallel to a line which itself is perpendicular to the lens optical axis and which lies in the guide slot plane of symmetry. The lens-mirror assembly fits concentrically within an annular lens holder which can rotate within a base housing, the housing itself comprising a vacuum cone which in practice is pneumatically held closely against the anterior surface of an eye. Thus, the lens-mirror assembly is rotatable within the base housing (Le., substantially about its optical axis). In certain embodiments, two eccentrically placed rotatable support rings allow fine lateral position adjustments of the lens-mirror assembly/holder within the base housing to bring the lens-mirror assembly optical axis into substantial collinearity with the visual axis of the eye on which the lens is placed. When locked to the base, the eccentric support rings act to maintain the instrument in a fixed position relative to the eye's visual axis while still allowing rotation of the lens-mirror assembly (in its holder) substantially around its optical axis. However, in certain other embodiments, the eccentrically placed rotatable support rings are simply eliminated and lateral positioning adjustments are made to the lens-mirror assembly during its initial placement on the eye by reference to corneal landmarks established with the aid of an optic zone marker. Rotation of the lens-mirror assembly (and its holder) within the base housing is limited by detents (e.g., ball in groove or clips) which may be in either the base or the holder. The detents can preferably be engaged to allow easy rotation of the lens- mirror assembly to any of a predetermined number (preferably 16) of accurately- determined rotational orientations, or released to allow substantially free (Le., sliding) rotation of the lens-mirror assembly. Establishment of predetermined rotational orientations allows a lens-mirror assembly having a scalpel guide slot to be quickly adjusted to guide one or more RK incisions. On the other hand, substantially free i 25 sliding rotation of a lens-mirror assembly allows a tangentially-oriented knife in the guide slot to make arcuate corneal incisions, preferably by rotating the lens-mirror assembly between mechanical stops (e.g., slots limiting holder rotation with respect to base, or a circumferential guide slot attached to the base which may be engaged by a removable stop pin which projects from the holder). Before an incision is made, the optical axis of the lens-mirror assembly is preferably aligned with the visual axis of the eye by adjustment of the eccentric alignment rings or by initial positioning (prior to application of a partial vacuum to the vacuum cone). Arcuate and/or radial incisions are then made manually with a scalpel, using the slot as a guide. One or two adjustable spacer blocks locked within the guide slot preferably limit scalpel travel within the guide slot on radially-directed incisions, or the cuts can be initiated by the surgeon at a premarked optic zone. For arcuate incisions, and one or more detents, stop pins, guide pins, and/or circumferential slots preferably limit rotation of the lens-mirror assembly substantially about its optical axis as described above. Because of the requirement for rotation of the lens-mirror assembly on the cornea during arcuate incisions, such incisions are preferably made before radial incisions to avoid the corneal distortion likely to follow if the incisions are made in the reverse order. The scalpel blade is also preferably preset to the approximate penetration depth desired before beginning the incision. Real-time control (adjustment) of the incision depth as the incision progresses is achieved by visually monitoring the blade and incision (substantially in cross- section) through the lens-mirror assembly and making blade-depth adjustments as needed to the scalpel which holds the blade. Achievement of the desired change in blade incision depth is confirmed visually by noting depth markings on the blade in relation to the anterior corneal epithelial surface, and also by noting the relative positions of the anterior corneal epithelial surface, the posterior corneal endothelial surface, and the blade tip within the incision. Visual estimation of the corneal remainder thickness (Le., the thickness of cornea between the blade tip and the corneal endothelial surface which is not incised) is facilitated by simultaneous visualization of the (posterior) corneal endothelial surface of the cornea, the blade tip, and the distance separating the two (again, substantially in cross-section), the visualized distance then being compared to known blade depth distances marked on the blade. Note that because of the distance scale visible on the blade which is adjacent to the corneal remainder, accurate estimation of corneal remainder thickness is possible even if the blade is not viewed from a point orthogonal to its longitudinal axis (which is substantially collinear with the scalpel longitudinal axis). A source of illumination allows the surgeon to use the mirror surfaces to constantly visually inspect the depth of the incision during the entire incision procedure. Because the lens material has a higher index of refraction than the tear film-cornea, illumination of the cornea from a light source from above the lens- mirror assembly would tend to produce substantial internal reflection as viewed through the internal mirror. However, light from a source above (Le., anterior to) the lens-mirror assembly which enters the scalpel slot has been found experimentally to penetrate the slot so as to provide tissue illumination sufficient to practice the invention. Other preferred embodiments may have the source of illumination beneath the lens-mirror assembly. To accomplish this retrolens illumination, one or more local light sources (e.g. , light-emitting or laser diodes) may be placed substantially adjacent the eye-contact (concave) surface(s) of the lens-mirror assembly and/or its holder. Alternatively, a fiber optic cable may pass through the lens-mirror assembly holder and/or the lens-mirror assembly itself at approximately a 30 degree angle with the optical axis (and preferably behind a mirror if within the lens), with its tip positioned near the limbus to produce sclerotic scatter type internal corneal illumination. Any suitable light source (e.g., a halogen light box or laser) may be used to introduce light to the fiber optic, although light of one or more particular wavelengths (or wavelength bands) may be preferred if used in conjunction with visibility-enhancing dyes on the cornea and/or fluorescent or phosphorescent-appearing portions of the scalpel knife. Additional light (of selected wavelengths, as above, in certain embodiments) may also be conducted to the site of the incision by an actively illuminated scalpel in which a fiber optic is optically coupled to conduct light through at least a portion of the scalpel handle and to the scalpel blade, which preferably comprises gem (e.g., diamond). The visibility of depth markings etched or otherwise applied to the surface of the (preferably diamond) blade may be enhanced by light transmitted from an outside source by the fiber optic within the scalpel handle. With both the blade and corneal surfaces illuminated as described, the effective depth setting for the blade may be estimated by (visual and/or scanned beam) inspection and thereafter adjusted as required, either manually or automatically. In one preferred embodiment of the lens-mirror assembly having a radial guide slot and a (curved superior) viewing surface on one side of the radial slot utilizes both refractive and reflective surfaces to substantially compensate and thus reduce optical aberrations to image the cornea effectively in cross-section. Transilluminated light carried to a diamond blade by a fiber optic and light passing through the scalpel guide slot from an external (anterior) source illuminate the corneal endothelial surface, blade and corneal stroma by a combination of sclerotic scatter and direct illumination. Light rays traverse the corneal epithelium and tear film and are refracted at the posterior concave surface of the lens-mirror assembly. Light rays that reach the mirrored surface are reflected toward the upper surface of the lens. All other rays are totally internally reflected. Light rays reaching the upper lens surface are again refracted by the curved anterior (viewing) surface, which corrects for astigmatic error induced by the posterior concave refracting surface. The image formed above the anterior surface of the lens is viewed by the optics of the surgical microscope. In the same preferred embodiment, the (plane) viewing surface on the other side of the slot utilizes only reflecting surfaces to image the cornea adjacent the knife slot substantially in cross-section. Light rays pass in both directions through this portion of the lens-mirror assembly, and illumination is provided to the corneal incision site by light passing through the slot from a light source above (Le., anterior to) the anterior surface of the lens. In addition to light passing through the lens and slot to the cornea, transilluminated light conducted to the knife blade by a fiber optic illuminates the blade and corneal endothelium. Light rays from the corneal stroma traverse the corneal epithelium and enter the tear film-saline media with virtually no refraction because of similar refractive indexes. Light enters the slot aperture in the posterior surface of the lens and is reflected at the mirrored surface. The center portion of the lens (Le., the slot) is a fluid filled space, confluent with the tear film through the posterior (slot) aperture mentioned above. Light rays leaving the internal mirrored surface continue upward toward the anterior lens surface, which is plane on one side of the slot; light rays are thus not refracted or reflected when they present normal to the plane surface. After exiting the anterior lens surface, the optics of the surgical microscope above image these rays for viewing. Surgical Procedure The visual axis of an eye on which RK is to be performed is determined and marked with a 2.5mm marker, aligning the astigmatic mark and the coincident plus axis of the astigmatic error. An optic zone marker of the calculated (desired) size is then used concentric to the centering mark. Circumferential guide slots reversibly locked or otherwise fixed to the base housing are aligned with the aligning astigmatic mark on the cornea. At the same time, the optical axis of the lens-mirror assembly is aligned with the centering marks on the cornea. Vacuum is applied to a vacuum cone in the base housing, thereby affixing the base to the perilimbal tissue. Care should be taken to assure that the posterior surface of the lens-mirror assembly is gently tensioned and in approximation to the corneal epithelium. Irrigation with balanced saline solution can be performed if necessary through the lens-mirror assembly slot. It is desirable to perform needed arcuate incisions first to avoid excessive rotational distortion of the cornea by the rotating lens-mirror assembly, which can occur after radial incisions have been completed. To perform an arcuate incision, a detent ball or clip reversibly locked to the base housing is disengaged to allow free rotation of the lens-mirror assembly holder. The lens-mirror assembly is rotated so as to align the center radial position of the knife guide slot with the aligning mark of the arcuate guide slot of the guide housing to be used. The knife positioning plate of appropriate optic zone is positioned on top of the keratoscopic lens, being sure to interlock it with knife (scalpel guide) slot. The keratoscopic lens should be rotated such that the arcuate stop peg is engaged into the arcuate slot guide as the plate is placed onto the top of the lens. The keratoscopic lens is rotated in either direction until its rotation is limited by the engaged stop peg. The transilluminated diamond knife is placed within the arcuate guide plate and knife slot with the blade retracted. The knife is oriented in the slot in a transverse position to the knife slot in the lens. The blade is advanced as the knife platform is pushed into contact with the corneal epithelium. Under direct visualization of the blade image in the keratoscopic lens, the diamond blade is observed in its descent into the corneal tissue. A slight balloting movement of the knife helps seat the blade. When the tip of the blade reaches a point of penetration as evidenced by the space between the tip of the blade and the projected reference light on the corneal endothelium directly beneath it. The lens-mirror assembly is then rotated between the confines of the arcuate guide slot with the knife and blade in the position described above. The knife is retracted after the incision is made with the blade extended. The arcuate stop peg is disengaged by lifting the plate, and the lens slot is rotated 180°, if an opposing arcuate incision is required. If a second, opposing arcuate incision is required, the arcuate stop peg is again engaged at the new position. The lens is turned in either position until limited by the arcuate slot guide. The knife is carefully reintroduced into the arcuate guide-knife slot and the platform pressed into contact with the corneal epithelium. Blade depth adjustments are performed while again visualizing the blade penetration depth in the optical system. The arcuate incision is again performed by maintaining this knife blade configuration and rotating the keratoscopic lens between the arcuate guide limits. The knife blade is retracted. The arcuate guide plate is then removed. The detent mechanism is adjusted to engage detent meridian indexing. The lens is rotated until the first radial index position is reached. The knife is reintroduced into the slot with the knife blade retracted. With the knife platform pushed into contact with the corneal epithelium, the diamond blade is positioned by direct visualization in the slot to enter the cornea at the border of the optic zone mark, previously made on the corneal surface. The diamond blade should be oriented to cut only in a centrifugal direction. The diamond blade is advanced into the corneal tissue by adjusting the blade depth under optical visualization of the blade in the tissue. Again, the gap between the tip of the diamond blade and projected reference light on the corneal endothelium should indicate appropriate blade penetration into the cornea. Again, a rocking or balloting motion of the knife will help to maximally seat the diamond blade within the tissue. The knife is drawn radially along the slot while maintaining contact of the knife platform with the corneal epithelium. When the terminus of the keratoscopic lens slot is reached, the knife is retracted in the slot. It is preferable to place the first radial incision in the thinnest meridian of the cornea. The keratoscopic lens slot can then be indexed to the next desired radial meridian and the procedure is repeated. It is preferable to rotate the lens counter-clockwise to avoid visualization across a previously made radial incision. If successively thicker meridians are incised, the diamond knife can be left extended and the depth of penetration adjusted at the optic zone mark under optical visualization as previously mentioned. Once all radial incisions are completed, the vacuum is terminated and the guide device is removed. The radial and arcuate incisions are checked for depth and irrigated. Enhancing Radial Incisions The visual axis marker is used to mark the visual axis of an eye. This impression should be visibly oriented to the central lens mark and central to the tips of the previously placed radial incisions. The calculated optic zone marker should be used concentrically around the visual axis mark to mark the desired optic zone on the corneal marker. The detent mechanism is engaged while maintaining centrational orientation of the keratoscopic lens to the alignment impression described above. The keratoscopic lens with the alignment check in place, along with the guide housing, is rotated around the axis of centration until the alignment mark of the alignment check is perfectly aligned along its length with the radial incision. Vacuum is applied to affix the device to the perilimbal structures, while assuring good contact of the keratoscopic lens undersurface to the corneal epithelium. The lens slot should then be indexed 360° in 90° increments. At each increment the alignment mark of the alignment check should align precisely with the radial incision beneath the lens. If any required arcuate incisions are placed during the enhancement, a further verification step is necessary. With the arcuate guide alignment mark aligned with the plus axis of the astigmatic refractive error, detent indexing is disengaged. The arcuate stop peg is engaged into the desired arcuate guide slot, as the keratoscopic lens is rotated in both directions to subtend the maximal arc between the arcuate guide limits. As the lens is rotated, the alignment mark of the alignment check should not pass over any previously placed corneal incisions beneath the lens. If it is noted that the intended arcuate incisions will intersect a previously placed radial corneal incision, a smaller subtended arcuate slot must be selected and reve fied with this process. If this is not possible, the lens and guide device must b, repositioned and again verified. The arcuate incisions are then performed as described in the general procedure. The alignment check is removed and the knife is positioned and its blade set at the optic zone mark as described in the general procedure. The length of the radial incision along the slot can be varied as to intersect or partially or completely retrace the original radial incision depending on the adequacy of its depth. This alignment procedure can be modified to realign the device in cases where it is inadvertently shifted before completion of a procedure. However, in addition to aligning the alignment mark to completed radial incisions, the alignment mark can be used to ensure that intended radial incisions will not intersect previously made arcuate incisions. This verification of alignment should occur after reapplication of vacuum and prior to placement of any consecutive incisions. Illustration of Preferred Embodiments Referring to the Figures, a preferred embodiment of the invention comprises an instrument which includes a slotted lens-mirror assembly 2, having an optical axis XX which is alignable with the visual axis of an eye and positioned so that an inferior, concave spherical surface 6 having a radius of curvature ranging from about 7.4 to about 10.0 mm can be placed on the anterior surface of a cornea which consequently conforms substantially to the shape of inferior surface 6. Embodiments of lens-mirror assembly 2 having a substantially coucave inferior surface 6 are designated keratoscopic lenses. The lens-mirror assembly 2 is preferably formed of a single piece of (preferably injection-molded) plastic. Alternatively, the lens-mirror assembly may be formed of a lower lens component (not shown) in the general shape of an inverted, truncated cone and an upper lens component (not shown) permanently bonded to the lower lens component. The curved refracting surface of the right side 7 of the superior surface as shown in Figure IB terminates at a substantially planar substantially vertical surface 8 substantially parallel to the optical axis XX. It should be noted that the portion of lens-mirror assembly 2 which lies to the left of surface 8 in Figure IB has a substantially planar superior surface 9, which may optionally be a curved refracting surface instead. Radial guide slots 16,18, which are best shown in Figure IB, combine to create an opening through lens-mirror assembly 2, slots 16,18 functioning as a scalpel guide for effecting radial surgical incisions in the cornea as the scalpel is moved along the radial slots. Viewed from the top of lens- mirror assembly 2, slot 18 is wider than slot 16 but has the same plane of symmetry (that is, a plane containing the optical axis XX). A substantially planar reflective surface 10 is positioned in the right portion of lens-mirror assembly 2 as seen edge-on in Figure IB beneath the curved upper surface 7. The reflective surface 10 may be formed as illustrated by making the reflective surface itself an air interface on lens-mirror assembly 2, thus providing for total internal reflection (or TIR) at surface 10. Alternatively, a mirrored (e.g., aluminized) surface may be embedded within lens-mirror assembly 2 in the position of surface 10 and with the same orientation (Le., facing axis XX and inclined at an angle of 56.5° relative to axis XX). A bottomless reservoir (not shown) communicating with inferior surface 6 (and in which an optical coupling solution can be stored) may optionally be provided in the left portion of the lens-mirror assembly 2 as seen in Figure IB. Another optional element in lens-mirror assembly 2 is a second mirror (not shown) having a TIR reflecting surface as above or a mirrored aluminized surface facing axis XX and inclined at an angle of 53.74° relative to axis XX. It should also be noted that the radial slot 16 is relatively narrow but sufficiently wide to accept in a close fit the knife platform 221 of scalpel 200 (see Figure 10), allowing sliding movement of the scalpel 200 substantially exclusively in a radial direction. However, the wider slot 18 is provided immediately above and symmetrically disposed on either side of the more narrow slot 16 to accommodate a portion of knife slot guide 104 (Figure 10), the transition from the width of slot 18 to the narrower slot 16 acting to limit travel of the knife slot guide (and thus the knife platform 221) toward the cornea. The mirror 10 (and the optional second mirror, if present) permit the surgeon to visually monitor the incision as it is being made through a microscope (not shown) looking downwardly through lens-mirror assembly 2 to facilitate substantially real-time control of corneal penetration depth of the tip 101 of scalpel blade 102 during radial or arcuate corneal incisions. It should be noted that reflecting surface angles other than the 53.74° and 56.5° described above may be used without departing from the scope of the invention, provided that the angle selected, in combination with the other optical parameters of lens-mirror assembly 2, allows the surgeon to visually monitor the incision. The lens-mirror assembly 2 may optionally have a tubular bore or hole (not shown) extending from the superior surface 7 to the concave inferior surface 6 which can hold a fiber optic (not shown) having its superior end coupled to a light source (not shown) and having its inferior end substantially contiguous with surface 6 for illuminating the cornea with scattered light during a surgical procedure. The lens-mirror assembly 2 fits telescopically within a ring-like lens-mirror guide assembly 24, which is represented in Figure 1C as the combination of structures which remain if the lens-mirror assembly 2 of Figure ID is removed from Figure lC. Guide assembly 24 comprises a keratoscopic lens holder 40 (Figures 3A and 3B) which in turn comprises a female key-way-like slot 42 to accept the lens-mirror assembly 2 and in which the lens-mirror assembly 2 is restrained by an overlapping flange 44 and a vertical bearing surface 45. A cornea (not shown) is preferably held closely proximate to concave surface 6 of lens-mirror assembly 2 by maintaining a partial vacuum in vacuum cone 56 which is formed by the spaced-apart upper and lower conical portions 51,61 of the upper and lower guide assembly housings 50,60 respectively. Arcuate cuts on the cornea (not shown) are made by rotating lens-mirror assembly 2 within lens-mirror guide assembly 24 substantially around optical axis XX as necessary to move a scalpel (Figure 10) within slot 16 along an arcuate path on the cornea. Radial cuts are preferably preceded by rotating lens-mirror assembly 2 into any desired one of 16 radial meridian positions which are easily located by clip and notch assemblies the details of which are discussed hereinafter. The top surface of upper guide assembly housing 50 comprises a plurality of substantially equidistantly spaced depressions 52 in a substantially circular pattern. Preferably 16 in number, each depression 52 is adapted for partially receiving spring- loaded ball 12 of detent pin 14. Detent pin 14 is threaded into detent pin hole 41 in keratoscopic lens holder 40 so that detent pin 14 may be advanced toward or backed away from contact with upper guide assembly housing 50. When detent pin 14 is backed away from contact with upper guide assembly housing 50, spring-loaded ball 12 is prevented from being completely expelled from detent pin 14 under the force of spring 15 by swaging detent pin 14 proximate the area where ball 12 would emerge. Such swaging, however, is carried out so as to allow at least a portion of spring-loaded ball 12 to protrude from detent pin 14 when detent pin 14 is backed away from contact with upper guide assembly housing 50. Thus, when detent pin 14 is advanced toward contact with upper guide assembly housing 50, spring-loaded ball 12 will make the first (and preferably the only) contact with upper guide assembly housing 50. With detent pin 14 thus adjusted so that spring-loaded ball 12 is partially received into a depression 52, keratoscopic lens holder 40 is then releasably rotationally coupled to upper guide assembly housing 50 in any one of 16 possible (detent) positions of rotation with respect to vacuum cone 56. With detent pin 14 thus adjusted, lens-mirror assembly 2 can be rotated with respect to lens-mirror guide assembly 24 substantially around axis XX to any desired one of 16 detent positions with respect to a cornea (not shown) proximate to concave spherical surface 6. Each detent position is one in which one depression 52 is aligned with spring-loaded ball 12 of detent pin 14 which is threaded through hole 41 in Keratoscopic lens holder 40. With detent pin 14 backed away so that spring-loaded ball 12 no longer contacts any part of upper guide assembly housing 50, the lens-mirror assembly 2 can be rotated freely with respect to lens-mirror guide assembly 24 (and thus with respect to a cornea to which the lens-mirror guide assembly 24 may be coupled via a partial vacuum in vacuum cone 56. Thus, with keratoscopic lens holder 40 in a detent position with respect to upper guide assembly housing 50, any corneal incisions made by movement of scalpel 200 (Figure 10) in a radial direction along slots 16,18 will likewise be in fixed relationship with depressions 52. That is, each detent position will substantially prevent accidental rotation of keratoscopic lens holder 40 and slot 16 with respect to vacuum cone 56 (which is fixed to the eye during the course of a corneal incision). Thus, the rotational detent positions of slots 16,18 determined by depressions 52 are used to properly position slots 16,18 for making desired radial incisions. However, the making of arcuate incisions is more complicated since arcuate incisions preferably do not intersect the radial incisions and would preferably be restricted to that portion of a cornea between adjacent radial incisions. When an operation includes the making of arcuate incisions as well as radial incisions, the radial incisions are usually made only at radial positions in alignment with every other depression 52, for a total of 8 substantially equally spaced radial incisions in the preferred method described herein. Restriction of the arcuate incisions to the portion of the cornea between these 8 radial incisions is effected by providing a plurality of arcuate guide slots 1,32,33,34 in the arcuate slot ring 30 of the lens-mirror guide assembly 24. Arcuate slots 32 and 34 each subtend a single-slot angle measured at axis XX that is less than the angle subtended at axis XX by any two adjacent depressions 52 (Le., less than 22.5 degrees in the above embodiment). Analogously, arcuate slots 31 and 33 each subtend a double-slot angle measured at axis XX that is less than the angle subtended at axis XX by any uninterrupted group of 3 depressions 52 (Le., less than 45 degrees in the above embodiment). Thus, arcuate slots 31 and 33 will be used to guide arcuate incisions in the case above where 8 substantially equally spaced radial incisions are also desired. To guide the radial incisions of the above example, an arcuatt __jide piece 70 is inserted within slot 18 of lens-mirror assembly 2 with the arcuate guide piece c " εnted so that protrusion 72 falls within either arcuate slot 31 or arcuate slot 33 of arcuate slot ring 30. The width W of arcuate guide piece 70 (Figure IF) is preferably dimensioned so as to allow a close sliding fit of arcuate guide piece 70 within the width of W of slot 18 (Figure IB). Additionally, end-contact surfaces 74,75 of arcuate guide piece 70 are dimensioned and positioned so as to obtain substantially firm and uniform contact with corresponding slot 18 surfaces 74',75' respectively when arcuate guide piece 70 is firmly inserted in slot 18 as above. With arcuate guide piece 70 thus inserted in slot 18, and with protrusion 72 extending into either slot 31 or slot 33, the possible range of rotational motion of lens-mirror assembly 2 (held substantially fixed to keratoscopic lens holder 40) with respect to the upper guide assembly housing 50 (and thus with respect to a cornea to which the upper guide assembly housing 50 is vacuum coupled) is limited to less than about 45 degrees. The exact value of allowed rotational motion will be seen by those skilled in the art to be easily predetermined through specification of the width of arcuate slots 31,33 and/or the dimensions of protrusion 72. Thus, by using the scalpel 200 substantially transversely fixed within guide slot 16 at a desired radial position by arcuate guide piece 70, it is possible to make an arcuate incision by rotation of the keratoscopic lens holder 40 (within lens-mirror guide assembly 24) so that the arcuate incision extends between, and yet is distinct from and not intersecting, adjacent radial corneal incisions. Note that the radius of curvature of any such radial incision will be a function of the radial position of scalpel port 76 with respect to axis XX. This radial position may be conveniently indicated, for example, by parameter Z (Figure IE) which represents the radial distance of the scalpel port 76 from surfaces 74,74'. Note that scalpel port 76 is substantially rectangular in cross-section to facilitate fixing of scalpel 200 in a substantially transverse orientation with respect to a radial line from axis XX. Note also that as parameter Z increases, angle α also increases in order to maintain scalpel 200 substantially perpendicular to a corneal surface being incised. It will be apparent from the foregoing that the lens-mirror assembly 2 and the keratoscopic lens holder 40 must be capable of being rotated to a desired position in which radial slot 16 is properly aligned for effecting a radial incision and the lens holder 40 (and thus the lens-mirror assembly 2, which is a keratoscopic lens in this example) is reversibly locked into position for the radial incision procedure (in this example with the threaded detent pin 14). Similarly, keratoscopic lens holder 40 must be capable of rotating with respect to the cornea and the upper guide assembly housing 50 during an arcuate incision procedure between the limits defined by slots 31,32,33,34 of the arcuate slot ring 30 as discussed above. The means for effecting and permitting the necessary movement and locking of the keratoscopic lens holder 40 and the associated lens-mirror assembly 2 is schematically illustrated in Figures 1A, IB, 4A and 4B. However, those skilled in the art will recognize that analogous combinations of mechanical stops and detents may be used to accomplish substantially the same objectives. Note that in use of the lens-mirror guide assembly, a vacuum source (not shown) is connected to a vacuum port comprising a vacuum port upper portion 58 (Figures 4A and 4B) and a vacuum port lower portion 68 (Figures 5A and 5B) to provide a vacuum to an annular chamber vacuum cone 56, thereby holding the device onto the sclera of the patient. Vacuum cone 56 becomes usable when upper guide assembly housing 50 and lower guide assembly housing 60 are sealingly assembled, leaving vacuum port upper portion 58 and vacuum port lower portion 68 substantially aligned and communicating with vacuum cone 56. The scalpel 200 illustrated schematically in Figure 10 provides means for a surgeon to adjust the effective depth of incision of the point 101 of a substantially transparent surgical diamond blade 102 used in corneal surgery. Note that in the illustration of Figure 10, physical proportions of scalpel 200 are altered somewhat to improve clarity in representations of relatively small structures such as the blade 102 and fiber optics 230 and 232. Elongated hollow scalpel shaft 222 is adjustably coupled to knife slot guide 104 via screws 119 which contact shaft 222 proximate its first end. Adjustment of the longitudinal position of knife slot guide 104 on scalpel shaft 222 has the effect of predetermining the extent to which knife platform 221 extends beyond knife slot guide 104 because knife platform 221 is firmly coupled (as by welding) to knife platform shaft 220, and knife platform shaft 220 is in turn firmly coupled to (and may be regarded as a part of) scalpel shaft 222. The extent to which knife platform 221 extends beyond knife slot guide 104 determines how far knife platform 221 will extend below slot 16 (and therefore depress any corneal surface which may be adjacent concave inferior surface 6) when scalpel 200 is inserted in scalpel slot 18 of lens-mirror assembly 2. Note that visualization of intracorneal structures is preferably accomplished with the present invention when the corneal epithelial surface is slightly depressed (dimpled) below the concave inferior surface 6 of the lens-mirror assembly 2. Knife platform 221 and knife platform shaft 220 are dimensioned to fit within slot 16 of lens-mirror assembly 2 in either a radial or transverse orientation without interference. Knife platform shaft 220 is firmly coupled to scalpel shaft 222 to prevent both relative rotation and relative longitudinal movement. Analogously, scalpel shaft 222 is firmly coupled to knife slot guide 104 to prevent both relative rotation and longitudinal movement. Knife slot guide 104, in turn, preferably has a substantially rectangular cross-section. The opposing lateral sides of slot guide 104 are spaced to allow a close sliding fit during radial movement of slot guide 104 within slot 18 (which has a width of approximately W as in Figure IB). For arcuate incisions, on the other hand, scalpel 200 and slot guide 104 are rotated 90 degrees about the scalpel longitudinal axis LL, after which slot guide 104 is inserted in a close sliding fit into scalpel port 76 of arcuate guide piece 70 to allow scalpel 200 to be stabilized in a substantially transverse orientation in slot 18. Note that knife blade 102 is preferably glued (as illustrated) to fiber optic 232 to comprise local fiber optic means, and substantially rigid angular blade supports 122 and fiber optic 232 are bonded with epoxy 216 to displacement member 228 to comprise blade adjustment means within knife platform 221 and knife platform shaft 220. Knife platform 221, knife platform shaft 220, and displacement member 228 preferably have substantially analogous non-circular cross-section (each preferably being substantially square) to substantially eliminate any tendency of blade 102 to rotate about the longitudinal axis LL of scalpel 200 within knife platform 221 and knife platform shaft 220. Displacement member 228 extends across fiber optic gap 227 and is firmly bonded to fiber optic 230. Fiber optic 230, in turn, couples linear displacement motor 266 (the displacement means) to displacement member 228 and thence to blade supports 122 and blade 102 for imparting substantially longitudinal movement to the blade adjustment means with respect to scalpel shaft 222. Such longitudinal relative movement is for extending and retracting blade 102 with respect to knife platform 221 through knife platform slot 123 (thus longitudinally moving blade 102 with respect to the second end of shaft 222). Motor 266 thus adjustably couples knife platform tube 220 (and scalpel shaft 222) to fiber optic 230 and thus to linear displacement tube 228 so that adjustable longitudinal movement of tube 228 with respect to tube 220 (and with respect to scalpel shaft 222) may be obtained in either direction and for a predetermined distance by appropriate actuation of the (reversible) linear displacement motor 266. Because rotation of blade 102 with respect to the longitudinal axis LL of scalpel 200 is thus substantially prevented, and because the scalpel shaft 222 is firmly coupled to knife slot guide 104 which fits closely (and nonrotatingly) into radial slot 18 in lens-mirror assembly 2, scalpel 200 is thereby guided in making desired radial and arcuate corneal incisions without undesirable changes in blade orientation. During the incisions with a scalpel of the above embodiment, light is conducted from a source (not shown) via fiber optic 230 substantially coaxially through linear displacement motor 266 to light gap 227 (which protects the superior end 103 of blade 102 from damage due to accidental contact with other structures). Light gap 227 is of substantially fixed length, being enclosed within linear displacement tube 228 (as moved longitudinally by linear displacement motor 266). Regardless of the length of light gap 227, light passes from fiber optic 230 to fiber optic 232 and to (diamond) blade 102 for illumination of the incision being made by blade 102. Note that if blade 102 comprised opaque material (e.g., stainless steel), light entering fiber optic 232 from fiber optic 230 could be conducted toward knife platform 122 and could be allowed to emerge from knife platform slot 123 proximate to and substantially parallel to blade 102 to illuminate optical transmission media (e.g., corneal tissue) which blade 102 may be incising. The extent of travel of the scalpel along radial slots 16 and 18 is limited for arcuate cuts by one of a plurality of arcuate guide pieces 70 described above (Figures IE, IF and IG), each piece 70 being dimensioned so that scalpel port 76 facilitates (guides) placement of knife blade tip 101 in a cornea at one of a predetermined set of desired radial distances from axis XX. The choice of which radial distance is appropriate for a particular corneal incision is a clinical judgment made by the surgeon. Determination of Optical Correction Throughout the following discussion, frequent reference will be made to optical ray tracings which represent the functional characteristics of certain embodiments of the invention and (for comparison) representative lenses selected from those currently commercially available. Ray tracings are made using techniques well known to those of skill in the art and incorporated in a commercially available computer program entitled BEAM 4 OPTICAL RAY TRACER, copyright 1989 by Stellar Software, P.O. Box 10183, Berkeley, CA 94709. Figures 6A, 6B, and 6C schematically illustrate a simplified cross-sectional view of a lens-mirror assembly 2 of the present invention. Features illustrated include concave inferior surface 6, slot 16, reflecting surface 10, and curved superior surface 7. Also included is a representation of ray tracings originating from a point 600 μ deep in corneal tissue and at the edge of optical zones 3 mm, 5 mm, and 7 mm in diameter in Figures 6A, 6B, and 6C respectively. In order to compress the ray tracings to a convenient size, the rays are reflected from a superior surface 302 and are seen to come to a focus on or slightly above a plane 300 seen edge-on in the Figures and representing the image plane of the microscope (not shown) through which the images would normally be observed in clinical practice. Image plane 300 is also represented in Figures 7A, 7B and 7C, which schematically illustrate light intensity histograms associated with the microscope images represented in Figures 6A, 6B, and 6C respectively. Each square in Figures 7A, 7B, and 7C has dimensions of approximately 0.75 μ on each side, and the histograms all represent relatively low values (Le., < about 5 μ) of root-mean-square (RMS) image error due to aberrations. The RMS error is a measure of the presence of aberrations which tend to defocus or spread out images, making the images unsuitable for the accurate incision depth measurements desired in use of the present invention. To better appreciate the functional differences between the lens-mirror assembly 2 of the present invention and prior lenses for examination of the eye, one may refer to Figures 8A, 8B, and 8C, which represent the results of ray tracings calculated for the same image origin (Le., a point 600 μ below the lens inferior surface and at the same optic zone edges as in Figures 7A, 7B, and 7C respectively) except that for Figures 8A, 8B, and 8C, the lens analyzed by the BEAM 4 program was that of a Zeiss gonioprism. The intensity histograms represent RMS error of at least twice that for the lens-mirror assembly 2 of the present invention (Le., > about 10 μ). Note that the Zeiss gonioprism, like other prior lenses, has no scalpel slots 16,18. Since the slots 16,18 may be expected to reduce light intensity in any lens in which they appear (compared to the same lens without the slots), the intensity values represented in Figures 8A, 8B, and 8C could probably not be realized in any actual (slotted) Zeiss gonioprism. And other prior lenses may perform even worse that the Zeiss gonioprism when used to view intracorneal structures as the lens-mirror assembly 2 of the present invention is used. Indeed, another commonly-used prior lens (Le., the Roussel lens) is schematically (isometrically) illustrated in Figure 9, wherein 310 represents the edge of the cornea, 320 represents the edge of the Roussel lens in place on the cornea, 330 represents a reflective surface positioned as it would be in a Roussel lens, and 307 represents the superior surface of the lens. It is apparent that of the rays originating at a point 600 μ below the lens inferior surface and at the edge of a 7 mm optic zone which can enter the lens inferior surface, all (as illustrated) will pass inferior to the inferior edge of the reflective surface 330 and therefore will not exit the superior surface 307 or form an image of the point of origin at plane 300. Even though both the Zeiss gonioprism and the Roussel lens have a concave inferior lens surface, a superior lens surface, and a reflecting surface in common with the lens-mirror assembly 2 of the present invention, the prior lenses differ in the location, shape and angle of the reflecting surface with respect to the optical axis of the lens, as well as in the optical correction applied through alteration of the shape of the superior lens surface and the choice of optical coupling medium index of refraction preferably used with the lens. For example, the index of refraction of the coupling fluid to be used between a keratoscopic lens of the present invention and a cornea should preferably lie in the range of about 1.2 to about 4 (more preferably between about 1.336 and 1.397). To attain a clinical objective of the present invention, that of viewing substantially in cross-section intracorneal structures (e.g., the tip 101 of knife blade 102) generally located below the inferior surface 6, one must first choose to optimize reflective surface position, shape and tilt, as well as the desired optical correction (preferably as incorporated in superior surface 7). Choice of any set of these parameters to yield an image as described herein having an RMS error substantially less than 5 μ while maintaining clinically useful luminance in the image is the design method preferably used to define embodiments of the keratoscopic lens of the present invention. Rays emanating from an object such as the length of a meridional corneal incision are presented obliquely to the optical system, and are known to be refracted substantially asymmetrically to produce a distorted image with respect to the object. The more obliquely the rays impinge the refracting surfaces, the more aberration (e.g., distortion or astigmatism) of oblique incidence that is produced. Tilting a spherical lens surface at an oblique angle to simulate this off-axis presentation of rays to the optical system produces the same effect. Rays that are incident on a lens surface at an oblique angle do not come to a point focus, but are separated into two focal lines with a circle (actually an ellipse) of least confusion. The imaginary envelope that is tangent to every ray is called the caustic surface, which in meridional cross section becomes the caustic curve. The best image point will be somewhat along the caustic curve, wherever the central concentration of image rays is greatest. The astigmatism of oblique incidence is exemplified by the following: tilting a +10.00 diopter spherical lens 10 degrees induces 0.31 diopters of astigmatism and increases the spherical power 0J0 diopters; tilting the same lens 20 degrees results in 1.38 diopters of astigmatic error and an increase of 0.41 diopters of spherical power; and, tiling the same lens 30 degrees results in 3.35 diopters of astigmatic error and increases the spherical power 0.95 diopters. Amounts for other lens powers are proportional, and the signs would be negative for a minus lens. In deciding on an optimal index of refraction for the coupling medium, many complex considerations can be entertained. Ideally, the coupling medium would have the following characteristics: (1) nontoxic to the corneal epithelium, (2) inexpensive, and (3) a viscosity which allows easy application between the lens and cornea but maintains an effective optical couple despite the open adjacent knife slot. The refractive index should also allow full visualization of the knife blade within the corneal stroma through the optical system. Because the knife slot is positioned by necessity over the entrance point into the corneal epithelium, this area of the corneal epithelium is not visible within the lens optical system unless the cornea in this region is depressed downward away from the knife slot. The ideal is to displace the cornea minimally while still visualizing this area. The refractive index can vary widely depending on this downward displacement of the object. For example, if the point to be visualized were 1.743 mm beneath the inferior surface of the lens, the required coupling index of refraction would be at least 0.7986 (less than air). If the point to be visualized were 0.95 mm beneath the under surface of the lens, the required coupling index of refraction would be at least 1.099. Because of the open knife slot in the keratoscopic lens design as mentioned above, the most superior point in the approximated cornea visible without downward displacement is 0.743 mm beneath the under surface of the lens. Therefore, the cornea must be depressed this much to allow visualization of the epithelium; the range of effective indexes of refraction would then be those values greater than 1J5. Chromatic aberration occurs because the amount of refraction occurring at each refractive interface is wavelength dependent. Chromatic aberration is enhanced by the angle of oblique incidence just as with astigmatic error. Optimizing the refractive index of the lens material at the air interface can reduce the overall spectral chromatic aberration. Also, using dissimilar refractive indexes in the lens body and the upper corrective lens can create a doublet effect that reduces chromatic aberration. It should be understood that the invention is not limited to the specific device as shown and that the spirit and scope of the invention extends to obvious variations that will be apparent to those of skill in the art and should therefore be defined solely by the appended claims.

Claims: What is claimed is: 1. An illuminating lens-mirror assembly for visualizing images of intracorneal structures in a cornea proximate the lens-mirror assembly, the assembly comprising a keratoscopic lens comprising an optical axis, a concave inferior surface, a superior surface, at least one internal mirror, and optical compensation means to substantially correct aberrations in the images of intracorneal structures; and illuminating means coupled to said lens for illumination of the intracorneal structures in a cornea proximate said lens inferior surface. 2. The lens-mirror assembly of claim 1 wherein said illuminating means can provide substantially retroillumination for a cornea proximate said lens inferior surface. 3. The lens-mirror assembly of claim 1 wherein said illuminating means can illuminate a cornea proximate said lens inferior surface substantially by scleral scattered light. 4. The lens-mirror assembly of claim 1 wherein said illuminating means comprise at least one substantially monochromatic light source. 5. The lens-mirror assembly of claim 1 wherein said optical compensation means comprises mirror compensation means causing said at least one internal mirror to increase resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 6. The lens-mirror assembly of claim 5 wherein said mirror compensation means comprises a substantially toroidal reflecting surface. 7. The lens-mirror assembly of claim 1 wherein said optical compensation means comprises lens compensation means to increase resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 8. The lens-mirror assembly of claim 7 wherein said lens compensation means comprises a substantially cylindrical refracting portion of said superior lens surface. 9. The lens-mirror assembly of claim 7 wherein said lens compensation means comprises a substantially toroidal refracting portion of said superior lens surface. 10. The lens-mirror assembly of claim 7 wherein said lens compensation means comprises a substantially spherical refracting portion of said superior lens surface. 11. A lens-mirror assembly, comprising a lens having an optical axis, an inferior surface, a superior surface, and at least one internal mirror, wherein said at least one internal mirror comprises mirror compensation means to increase resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 12. The lens-mirror assembly of claim 11, wherein said mirror compensation means comprises a substantially toroidal reflecting surface. 13. A lens-mirror assembly, comprising a lens having an optical axis, an inferior surface, a superior surface, and at least one internal mirror, wherein said superior lens surface comprises lens compensation means to increase resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 14. The lens-mirror assembly of claim 13 wherein said lens compensation means comprises a substantially cylindrical refracting surface. 15. The lens-mirror assembly of claim 13 wherein said lens compensation means comprises a substantially toroidal refracting surface. 16. The lens-mirror assembly of claim 13 wherein said lens compensation means comprises a substantially spherical refracting surface. 17. A keratoscopic lens for visualizing images of intracorneal structures in a cornea proximate the keratoscopic lens, the lens comprising an optical axis, a substantially concave inferior surface, a superior surface, a scalpel slot extending through said lens from said inferior surface to said superior surface, at least one internal mirror, and optical compensation means to substantially correct aberrations in the images of intracorneal structures in a cornea proximate said inferior surface. 18. The keratoscopic lens of claim 17 wherein said scalpel slot is oriented substantially radially with respect to said lens optical axis. 19. The keratoscopic lens of claim 17 wherein said at least one internal mirror comprises mirror compensation means to improve resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 20. The keratoscopic lens of claim 19, wherein said mirror compensation means comprises a substantially toroidal reflecting surface. 21. The keratoscopic lens of claim 17 wherein said superior lens surface comprises lens compensation means to improve resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 22. The keratoscopic lens of claim 21 wherein said lens compensation means comprises a substantially cylindrical refracting surface. 23. The keratoscopic lens of claim 21 wherein said lens compensation means comprises a substantially toroidal refracting surface. 24. The keratoscopic lens of claim 21 wherein said lens compensation means comprises a substantially spherical refracting surface. 25. An illuminating lens-mirror assembly, comprising a lens having an optical axis, an inferior surface, a superior surface, a scalpel slot extending through said lens from said inferior surface to said superior surface, and at least one internal mirror; and illuminating means for illumination of optical transmission medium which may be proximate said lens inferior surface. 26. The lens-mirror assembly of claim 25 wherein said scalpel slot is oriented substantially radially with respect to said lens optical axis. 27. The lens-mirror assembly of claim 25 wherein said at least one internal mirror comprises mirror compensation means to improve resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 28. The lens-mirror assembly of claim 27, wherein said mirror compensation means comprises a substantially toroidal reflecting surface. 29. The lens-mirror assembly of claim 25 wherein said superior lens surface comprises lens compensation means to improve resolution in images formed substantially of light rays oblique with respect to said lens optical axis. 30. The lens-mirror assembly of claim 29 wherein said lens compensation means comprises a substantially cylindrical refracting surface. 31. The lens-mirror assembly of claim 29 wherein said lens compensation means comprises a substantially toroidal refracting surface. 32. The lens-mirror assembly of claim 29 wherein said lens compensation means comprises a substantially spherical refracting surface. 33. A scalpel, comprising an elongated hollow scalpel shaft having a longitudinal axis, a first end, and a second end; blade adjustment means within and adjustably coupled to said scalpel shaft for extending and retracting a blade with respect to said shaft second end substantially parallel to said shaft longitudinal axis; a blade coupled to said blade adjustment means proximate said scalpel shaft second end; displacement means coupled to said blade adjustment means and said scalpel shaft to impart substantially longitudinal movement to said blade adjustment means with respect to said scalpel shaft for extending and retracting a blade with respect to said shaft second end substantially parallel to said shaft longitudinal axis; local fiber optic means for coupling electromagnetic radiation to said blade; and input fiber optic means for delivering electromagnetic radiation to said local fiber optic means, said input fiber optic means being spaced apart from said local fiber optic means. 34. The scalpel of claim 33 wherein said displacement means comprises a pneumatic motor. 35. The scalpel of claim 33 wherein said displacement msans comprises a pico motor. 36. The scalpel of claim 33 wherein said displacement means comprises a brushless direct-current motor. 37. The scalpel of claim 33 wherein said input fiber optic means comprises at least one laser diode. 38. The scalpel of claim 33 wherein said input fiber optic means comprises at least one remote fiber optic extending from said local fiber optic means to an electromagnetic radiation source outside of said scalpel shaft. 39. The scalpel of claim 33 wherein said blade comprises fluorescent material. 40. The scalpel of claim 33 wherein said blade comprises diamond. 41. The scalpel of claim 33 wherein at least a portion of said input fiber optic means is substantially coaxial with said displacement means. 42. A method for improving resolution in images formed substantially of light rays oblique with respect to the lens optical axis within a lens-mirror assembly as in claim 1, the method comprising configuring said at least one internal mirror with a reflective surface which is at least partly substantially toroidal. 43. A method for improving resolution in images formed substantially of light rays oblique with respect to the lens optical axis within a lens-mirror assembly as in claim 1, the method comprising configuring at least part of said superior lens surface to be substantially cylindrical. 44. A method for improving resolution in images formed substantially of light rays oblique with respect to the lens optical axis within a lens-mirror assembly as in claim 1, the method comprising configuring at least part of said superior lens surface to be substantially spherical. 45. A method for improving resolution in images formed substantially of light rays oblique with respect to the lens optical axis within a lens-mirror assembly as in claim 1, the method comprising configuring at least part of said superior lens surface to be substantially toroidal. 46. An optical measurement appliance to measure depth of a relative opacity in an optical transmission medium, comprising an electromagnetic radiation port to direct electromagnetic radiation into the optical transmission medium; an electromagnetic radiation receiver spaced apart from and coupled to said electromagnetic radiation port; and an electromagnetic radiation coupler for coupling to said electromagnetic radiation receiver at least a portion of electromagnetic radiation transmitted through said electromagnetic radiation port and through the optical transmission medium proximate said electromagnetic radiation port and said electromagnetic radiation receiver. 47. The optical measurement appliance of claim 46 wherein said electromagnetic radiation port comprises a laser diode array. 48. The optical measurement appliance of claim 46 wherein said electromagnetic radiation port comprises a fiber optic array. 49. The optical measurement appliance of claim 46, wherein said electromagnetic radiation coupler comprises a lens-mirror assembly as in claim 11. 50. The optical measurement appliance of claim 46, wherein said electromagnetic radiation coupler comprises a lens-mirror assembly as in claim 13. 51. The optical measurement appliance of claim 46, wherein said electromagnetic radiation coupler comprises a lens-mirror assembly as in claim 17. 52. An optical guide, comprising an optical measurement appliance as in claim 51; and a scalpel as in claim 33, said scalpel housing being slidingly and reversibly coupled through said scalpel slot to said lens-mirror assembly, and said blade adjustment means being coupled to said electromagnetic radiation detector. 53. A method of estimating depth of a relative opacity in an optical transmission medium sheet, the method comprising providing an optical transmission medium sheet comprising a first side and a second side, the relative opacity extending from said sheet first side toward said sheet second side, positioning an electromagnetic radiation port proximate said sheet first side to direct electromagnetic radiation comprising a plurality of distinct frequencies into said sheet first side proximate the relative opacity, electromagnetic radiation of each frequency of said plurality of distinct frequencies being located in depth from said sheet first side substantially separately within said sheet relative to electromagnetic radiation of any other distinct frequency from said radiation port; spacing an electromagnetic radiation detector apart from said electromagnetic radiation port, said radiation detector being sensitive to at least each of said plurality of distinct frequencies; optically coupling said electromagnetic radiation detector to said optical transmission medium sheet first side; positioning said electromagnetic radiation detector so that electromagnetic radiation of at least one but not all said distinct frequencies is attenuated in transmission from said electromagnetic radiation port through a region of the optical transmission medium containing the relative opacity and thence to said electromagnetic radiation detector; measuring electromagnetic radiation of each frequency of said plurality of distinct frequencies with said radiation detector; comparing said electromagnetic radiation measurements to identify each said attenuated distinct frequency; and correlating each said separate electromagnetic radiation depth location with each said attenuated distinct frequency to estimate depth of the relative opacity within the optical transmission medium sheet. 54. The method of claim 53 wherein at least a portion of electromagnetic radiation transmitted through said electromagnetic radiation port is internally reflected from the optical transmission medium sheet second side before entering said electromagnetic radiation detector. 55. The method of claim 54 wherein at least a portion of electromagnetic radiation transmitted through said electromagnetic radiation port is totally internally reflected from the optical transmission medium sheet second side before entering said electromagnetic radiation detector. 56. A method of performing radial keratotomy, the method comprising making at least one substantially radial, partial-thickness corneal incision while observing said incision using an illuminating lens-mirror assembly according to claim 1. 57. A method of performing radial keratotomy, the method comprising making at least one substantially radial, partial-thickness corneal incision while observing said incision using an illuminating lens-mirror assembly according to claim 11. 58. A method of performing radial keratotomy, the method comprising making at least one substantially radial, partial-thickness corneal incision while observing said incision using an illuminating lens-mirror assembly according to claim 17. 59. A method of performing radial keratotomy, the method comprising making at least one substantially radial, partial-thickness corneal incision while observing said incision using an illuminating lens-mirror assembly according to claim 25. 60. A method of estimating depth of a relative opacity established in an optical transmission medium, the method comprising providing an optical transmission medium; illuminating said optical transmission medium with electromagnetic radiation from at least one electromagnetic radiation port; optically coupling an electromagnetic radiation detector to said optical transmission medium, said radiation detector being spaced apart from said at least one electromagnetic radiation port; measuring baseline electromagnetic radiation received at said radiation detector; establishing a relative opacity in said optical transmission medium; measuring altered electromagnetic radiation received at said radiation detector; comparing said altered electromagnetic radiation measurement to said baseline electromagnetic radiation measurement to estimate depth of said relative opacity. 61. The method of claim 60 wherein said at least one electromagnetic radiation port provides substantially uniformly scattered electromagnetic radiation within said optical transmission medium. 62. The method of claim 60 wherein said relative opacity is established by a knife blade incising said optical transmission medium. 63. The method of claim 62 wherein said knife blade is substantially opaque to said electromagnetic radiation. 64. The method of claim 63 wherein said radiation detector is spaced apart from said knife blade. 65. The method of claim 62 wherein said knife blade is substantially transparent to said electromagnetic radiation. 66. The method of claim 65 wherein said radiation detector substantially detects electromagnetic radiation striking said substantially transparent knife blade. 67. The method of claim 60 wherein said electromagnetic radiation is substantially directed from said at least one electromagnetic radiation port toward said radiation detector. 68. The method of claim 67 wherein said optical transmission medium is substantially in sheet form. 71. A method of delivering electromagnetic radiation accurately within the stroma of a cornea, the method comprising applying the illuminating lens-mirror assembly of claim 25 to the cornea; incising the corneal stroma with the scalpel of claim 33 under direct vision through said illuminating lens-mirror assembly; injecting a delaminating fluid into the corneal stroma to separate the stroma into a plurality of stromal layers; choosing at least one of said separated stromal layers to receive electromagnetic radiation; and focussing electromagnetic radiation on said at least one chosen stromal layer to deliver electromagnetic radiation accurately within the stroma. 72. A method of effecting corneal trephination under direct visualization, the method comprising applying the illuminating lens-mirror assembly of claim 25 to the cornea; incising the cornea in a substantially circular pattern with the scalpel of claim 33 to effect corneal trephination under direct visualization through said illuminating lens-mirror assembly. 73. The method of claim 72 wherein said incising of the cornea is full-thickness. 74. The method of claim 72 wherein said incising of the cornea is partial-thickness.

Inventors: SMITH ALAN D, SMITH, ALAN, D.
Applicants: SMITH ALAN D, SMITH, ALAN, D.
Application year: 1994
Main classification: A61F9
IPC codes: A61B3, A61F9